An electromagnetic actuator for actuating a setting member has at least one electromagnet, which can be supplied with current by a control device, and an armature that is operatively connected with the setting member to be actuated and which can be brought out of a first set position, counter to the force of a restoring spring, into a second set position, in which the armature is in contact with the pole face of the electromagnet, when the electromagnet is supplied with current. If the electromagnet is set to be currentless, the armature falls back into its first set position.
The same applies for an electromagnetic actuator in which two electromagnets are disposed with spacing from one another, and between which the armature can be brought into contact with the one electromagnet in its first set position, and with the other electromagnet in its second set position, counter to a respective restoring spring, when the electromagnets are alternatingly supplied with current with the aid of a control device.
A critical factor in operating such electromagnetic actuators is that the armature be reliably brought into the set position defined by contact with the pole surface of the electromagnet, and that the armature be held securely by the electromagnet until the current to the electromagnet is switched off by the control device.
If, for example, restoring springs having a linear characteristic are used, as the armature approaches the pole surface of the electromagnet being supplied with current, a progressively-increasing magnetic force counteracts the linearly-increasing restoring force of the spring acting on the armature, so the moving speed of the armature increases as it nears the pole surface. A high impact speed of the armature at the pole surface is not only perceptible in an increased noise level, but in an extreme case, the high speed can cause the armature to "bounce" from the pole surface, which, in a favorable case, can result in multiple armature impacts until motionless contact has been achieved. In an unfavorable case, the bouncing can be so severe that the armature never achieves contact with the pole surface, but is moved back in the direction of the first set position under the influence of the restoring force. This effect can be somewhat diminished through the use of springs having a progressive characteristic, but a considerable surplus of magnetic force remains toward the end of the armature motion.
Through a corresponding control of the current supply as the armature approaches the pole surface of the electromagnet, it is possible to reduce the magnetic force proportionately to the distance from the pole surface, so the braking influence of the restoring force of the restoring spring can be better utilized and, accordingly, the armature can come to rest "gently" with reduced speed on the pole surface. Thus, not only is the noise level reduced, but at the same time the risk of bouncing is practically precluded.
To guide the current supply to the electromagnet via the control device so as to ensure reliable capture of the armature at the electromagnet, the motion of the armature must be detected and, in the same motion cycle, the magnetic force must be influenced. This is possible, for example, through the arrangement of an electrically-inductive sensor that detects the passing of the armature at a predetermined distance from the pole surface of the capturing electromagnet, so this sensor can trigger a signal that effects a reduction in the current supply to the electromagnet by a predetermined amount, and thus a reduction in the magnetic force. In this method, however, only the time at which the armature passes, but not its moving speed, can be detected. If, in addition to, for example, the force of the restoring spring counteracting the armature motion, additional forces occur in the same or opposite direction over the course of the motion, such as frictional forces from the armature guidance or stochastically-stipulated counter-forces acting on the setting member to be actuated, the flight speed of the armature can be reduced such that the armature would require the entire magnetic force to even come into contact with the pole surface of the electromagnet. If the magnetic force is reduced by a corresponding change in the current supply at the time when the armature passes the sensor, it is entirely possible that the magnetic force will not suffice even to capture the armature, so the armature will move back in the direction of its first set position.
This type of "static" detection of the armature position with a sensor is thus unsatisfactory, so attempts have been made to measure the actual armature speed with additional sensors and a time detection for obtaining a corresponding correction value to influence the current supply. It has also been proposed to derive the electrically-inductive reactions resulting from the approach of the armature to the electromagnet for correction signals, which will then be used to change the current supply, and thus change the magnetic force.